The invention relates to a magnetic resonance method for suppressing a signal from a chemical shift component in a magnetic resonance image of a body, the method comprising successive steps of situating the body in a stationary homogeneous magnetic field, irradiating the body with a 180.degree. chemical shift selective rf-pulse being frequency selective to the chemical shift component, applying a dephasing magnetic field gradient, and applying a delay time after the 180.degree. rf-pulse a spin-echo acquisition scheme for acquiring magnetic resonance signals from the body, the scheme comprising at least one phase-encoding gradient and the delay time being such that at the start of the spin-echo acquisition scheme a mean longitudinal relaxation component of said chemical shift component will be approximately zero, said successive steps being repeated a number of times, whereby the image is reconstructed by means of a Fouriertransformation of samples from the magnetic resonance signals.
The invention further relates to a magnetic resonance device for suppressing a signal from a chemical shift component in a magnetic resonance image of a body, which device comprises means for generating a stationary homogeneous magnetic field, means for irradiating rf-pulses to the body, modulating means for modulating the rf-pulses, receiving and demodulating means for receiving and demodulating excited magnetic resonance signals, sampling means for sampling the magnetic resonance signals and control means for controlling the means for irradiating rf-pulses, the modulating means, the receiving and demodulating means and the sampling means being such that the signal from said chemical shift component is attenuated and that a signal from other components is achieved, the device further comprising Fourier transform means for reconstructing an image from the sampled magnetic resonance signals.
Such a method and device are known from the Abstract, "Selective Partial Inversion Recovery (SPIR) in Steady State for Selective Saturation Magnetic Resonance Imaging (MRI)", Book of Abstracts, SMRM 1988, page 1042. Therein it is suggested to suppress fat signals in MRI by first applying a partial inversion, frequency selective rf-pulse to a body situated in a homogeneous magnetic field, then applying a dephasing gradient and finally applying a spin-echo scheme with a 90.degree. spatially selective rf-pulse and a non-selective 180.degree. pulse. Said partial inversion rf-pulse is an amplitude modulated pulse. The suppression is achieved by using a delay time between the partial inversion rf-pulse and the 90.degree. pulse such that the 90.degree. pulse is applied at the time of the zero-crossing of longitudinal relaxation of the chemical shift component and by dephasing the remaining transverse magnetization components. Transverse magnetization is dephased by the dephasing gradient applied after the frequency selective, partial inversion pulse. In said method the partial inversion rf-pulse has typical values of 120.degree.-130.degree. corresponding to a delay time of 30-40 ms for SE-fat suppression sequences. In vivo T1-relaxation for fat (in the order of 250 ms) is much shorter than for water. As a range for the flip angle of the partial inversion pulse angles of 90.degree.-180.degree. are specified. It should be noticed here that a 180.degree. inversion pulse will not give partial but full inversion. It is stated that for large rf-field inhomogeneities the partial inversion pulse should be increased to 180.degree. to make magnetization less sensitive for the inhomogeneities. It is found however that increasing of the amplitude modulated partial inversion pulse to 180.degree. still does not give satisfactory results; still unacceptable image artifacts remain. Said method is sensitive for inhomogeneities in the stationary magnetic field and in the rf-field. Where image detail is required of tissue surrounded by fat as is the case when imaging e.g. an optic nerve in a human head said fat suppression method will not give uniform fat suppression. To clearly visualize small detailed structures excellent fat suppression will be necessary. When e.g. a human head is imaged with said method, using a linearly polarized head-coil, giving rise to relatively large rf-inhomogeneities, said method does not give satisfactory results.